1. Field of the Invention
The present invention relates to a semiconductor device for an integrated injection logic cell and a process for fabricating the same. More particularly, the present invention is directed to a semiconductor device which realizes, for example, an integrated injection logic cell having a silicon nitride film, and a process for fabricating the same.
2. Description of the Related Art
A non-examined Japanese Laid-Open Publication No. 8-316,333 (corresponding to U.S. Pat. No. 6,008,524) discloses an example of an IIL (Integrated Injection Logic) cell which is realized by using an emitter-base self-alignment bipolar transistor in which a base electrode and an emitter electrode of the bipolar transistor are formed from a first semiconductor thin film and a second semiconductor thin film, respectively.
However, in this IIL cell, a diffused layer, which corresponds to a base region of a pnp bipolar transistor of the IIL cell, generally has a low impurity concentration as low as 1×1015 to 1×1016 counts/cm3. The reason for this is explained as follow. When the impurity concentration of the diffused layer is higher than the above mentioned value, an hfe, i.e., a current amplification factor of the bipolar transistor is lowered, so that an injection current which flows through the base region is difficult to flow and the operation of the IIL cell becomes difficult. On the other hand, if the impurity concentration of the diffused layer is lower than the above value, an operating speed of the IIL cell itself is disadvantageously lowered. For this reason, the impurity concentration of the diffused layer of the bipolar transistor is typically set within the above mentioned range. As mentioned above, however, resultantly the impurity concentration of the diffused layer has to be relatively low. Therefore, the property of the pnp bipolar transistor is very sensitive to the surface state, and particularly, non-uniformity in the properties among semiconductor chips remarkably occurs due to contamination and damage caused during the steps for processing a conductive layer and for processing a wiring. Further, a problem about the deterioration of the reliability often occurs.
In this situation, the present inventor has made extensive and intensive studies with a view toward solving the above-mentioned problems accompanying the conventional art. As a result, it has unexpectedly been found that, in a semiconductor device which comprises an integrated injection logic cell having a pnp bipolar transistor structure formed on a semiconductor substrate, wherein at least one of insulating films formed on a base region of the pnp bipolar transistor structure is comprised of a silicon nitride film. As the silicon nitride film advantageously prevents an occurrence of contamination on the surface of the base region of the pnp bipolar transistor, so that both the properties of the pnp bipolar transistor and the operation of the IIL cell can be stabilized.